Calling API commands

I'm working on a project which involves a bunch of external Dll. I have wrappers around these Dlls and have a bunch of API calls I would like to get at through a custom language, where would I start to look/ what challenges might I face if I were to try
and glue in custom commands in this way?

Have a look at this article: http://intellect.dk/post/Writing-a-calculator-in-C-using-Irony.aspx it shows how to add calls to selected math functions inside expressions, custom impl of FunctionCallNode